Noise reduction device, noise reduction method and image capturing device

ABSTRACT

A noise reduction device comprises a first signal reading-out unit for reading out a video signal of a target pixel for which a noise reduction is performed; a second signal reading-out unit for reading out video signals of external peripheral pixels arranged around internal peripheral pixels arranged at positions closest to the target pixel, the external peripheral pixels being arranged at positions closest to the target pixel in radial directions from the target pixel and outputting the same color signals as the video signal of the target pixel; a selection controller for judging presence of a correlation between the video signal of the target pixel and a video signal of each of the external peripheral pixel based on a relation between a signal value of the video signal of the target pixel and a signal value of the video signal of each of the external peripheral pixels; a signal calculator for calculating the signal value of the video signal of the target pixel by use of the signal values of the video signals of the external peripheral pixels; and a selector for selecting the signal value of the video signal of the target pixel determined by the signal calculator when it is judged by the selection controller that the number of the video signals of the external peripheral pixels having correlations with the video signal of the target pixel is large.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Applications P2004-94594 filed on Mar. 29, 2004and P2004-289324 filed on Sep. 30, 2004; the entire contents of whichare incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Filed of the Invention

The present invention relates to a noise reduction device and a noisereduction method, which reduces noise of a video signal by correctingsignal values of pixels based on a relation between a target pixel andeach of a plurality of peripheral pixels which are located around thetarget pixel. The present invention also relates to an image pickupapparatus having the noise reduction device. Particularly, the presentinvention relates to a noise reduction device and a noise reductionmethod which reduces noise of a video signal containing a color signal,and to an image pickup apparatus having the noise reduction device.

2. Description of the Related Art

A video signal obtained by image pick-up by use of a solid-state imagepickup device such as a CCD (Charge Coupled Device) and a C-MOS sensoris a signal in which noise is superimposed on pixels, due to variationsin characteristics among the pixels, influence of wiring resistances andthe like. Therefore, a noise reduction circuit to reduce the noisesuperimposed in the video signal output from the solid-state imagepickup device has heretofore been provided in a circuit at a rear stagebehind the solid-state image pickup device.

In most of such noise reduction devices, by use of each of the signalvalues of a plurality of peripheral pixels existing within apredetermined range, a center of which is a target pixel to be subjectedto a noise reduction processing, and by use of a correlation between thetarget pixel and each of the peripheral pixels, a signal value in whichnoises are averaged is obtained. This signal value is used as a signalvalue of the target pixel. At this time, presence of the correlationbetween the target pixel and each of the peripheral pixels is judged bycomparing the difference of the signal value between the target pixeland each of the peripheral pixels with a threshold value. Subsequently,the number of peripheral pixels correlated with the target pixel iscounted, and a sum of the signal values of peripheral pixels correlatedwith the target pixel and the signal value of the target pixel isobtained. An average value obtained by use of the number of peripheralpixels and the sum of the signal values of the pixels is used as thesignal value of the target pixel, whereby noises are removed.

In addition, as a technology using the signal values of the peripheralpixels correlated with the target pixel as described above, a noisereduction device has been proposed, in which a threshold value to judgethe presence of the correlation and an area where the peripheral pixelsused are included are switched based on the signal value of the targetpixel, any one of an average value and a variance for one frame, and anaverage value of high frequency components (see Japanese PatentLaid-Open No. 2003-179779).

SUMMARY OF THE INVENTION

However, in a video signal output from a solid-state image pickup devicein which plural kinds of color filters are provided, the same colorsignals are adjacent to each other for every two pixels. Accordingly,even when a correlation is actually high, the noise reduction processingmay be performed based on the premise that there is no correlation.Specifically, color filters respectively provided for directly adjacentpixels (adjacent to each other every one pixel) in the solid-statepickup device are different from each other in their kinds, so thatthese color filters have different light transmittances. Accordingly,even if the quantities of incident light are equal, the signal values ofthe video signals from the pixels directly adjacent to each other aredifferent. Therefore, it is difficult to perform an appropriate noisereduction processing.

In view of such a problem, an object of the present invention is toprovide a noise reduction device and a noise reduction method, which arecapable of performing noise reduction processing more appropriately. Inaddition, another object of the present invention is to provide an imagepickup apparatus having the noise reduction device.

In order to achieve the objects, a first aspect of the present inventionis to provide a noise reduction device reducing a noise superimposed onvideo signals including plural kinds of color signals, each of which isoutput by a pixel, which includes a first signal reading-out unit forreading out a video signal of a target pixel for which a noise reductionis performed; a second signal reading-out unit for reading out videosignals of external peripheral pixels arranged around internalperipheral pixels arranged at positions closest to the target pixel, theexternal peripheral pixels being arranged at positions closest to thetarget pixel in radial directions from the target pixel and outputtingthe same color signals as the video signal of the target pixel; aselection controller for judging presence of a correlation between avideo signal of the target pixel and a video signal of each of theexternal peripheral pixels based on a relation between a signal value ofthe video signal of the target pixel and a signal value of the videosignal of each of the external peripheral pixels; a signal calculatorfor calculating the signal value of the video signal of the target pixelby use of the signal value of the video signal of each of the externalperipheral pixels; and a selector for selecting the signal value of thevideo signal of the target pixel calculated by the signal calculatorwhen it is judged by the selection controller that the number of thevideo signals of the external peripheral pixels having correlation withthe video signal of the target pixel is large.

According to this aspect, each of the external peripheral pixels is onepixel away from the target pixel, and outputs the same color signal asthe target pixel. Further, based on the relation between the signalvalue of the video signal of each of the external peripheral pixels,whose color signals are the same, and the signal value of the videosignal of the target pixel, the selection controller judges whetherthere is the relation between each of the external peripheral pixels andthe target pixel. Therefore, the correlation between each of theexternal peripheral pixels and the target pixel can be comprehended moreaccurately. Accordingly, it is possible to perform the noise reductionprocessing more accurately.

In such a noise reduction device, the device may further include a thirdsignal reading-out unit for reading out video signals of internalperipheral pixels arranged around the target pixel and at positionsclosest to the target pixel in radial directions from the target pixel,wherein the selection controller may judge presence of a correlationbetween the video signal of the target pixel and the video signal ofeach of the external peripheral pixel based on the relation between thesignal value of the video signal of the target pixel and the signalvalue of the video signal of each of the external peripheral pixels andbased on a relation between signal values of the video signals of therespective internal peripheral pixels.

Based on a difference between the signal value of the video signal ofthe target pixel and the signal value of the video signal of each of theexternal peripheral pixels and based on a difference between signalvalues of video signals of the respective internal peripheral pixels,presence of the correlation between the video signal of the target pixeland the video signal of each of the external peripheral pixels isjudged. The difference between the signal values of the internalperipheral pixels is obtained by subtracting the signal values of thevideo signals of the internal peripheral pixels whose color signals arethe same.

Furthermore, in the signal calculator, by calculating a weighted sum ofthe signal values of the video signals of the external peripheralpixels, or by calculating a weighted sum where the signal value of thevideo signal of the target pixel is added to the signal value of thevideo signal of each of the external peripheral pixels, the signal valueof the video signal of the target pixel may be determined.

In such a noise reduction device, when the number of the externalperipheral pixels are provided by n (n: a natural number), the nexternal peripheral pixels are first through n-th external peripheralpixels, the n internal peripheral pixels are first through n-th internalperipheral pixels, and a direction of the k-th (k: a natural numbersatisfying 1≦k≦n) external peripheral pixel with respect to the targetpixel and a direction of the k-th internal peripheral pixel with thetarget pixel are the same, the device may further include a firstcorrelation value calculator for determining first to n-th maincorrelation values respectively indicating presence of the correlationsbetween the respective video signals of the first through n-th externalperipheral pixels and the video signal of the target pixel, based onresults obtained by subtracting the signal value of the video signal ofthe target pixel, which has been read out by the first signalreading-out unit, from the respective signal values of the video signalsof the first through n-th external peripheral pixels, which has beenread out by the second signal reading-out unit; and a second correlationvalue calculator for determining first through n-th sub-correlationvalues respectively indicating presence of the correlations between therespective video signals of the first through n-th external peripheralpixels and the video signal of the target pixel, based on a resultobtained by subtracting the signal values of the video signals of theinternal peripheral pixels existing at a position symmetrical to therespective first through n-th internal peripheral pixels around thetarget pixel from the signal values of the video signals of respectivefirst through n-th internal peripheral pixels, which have been read outby the second signal reading-out unit, and the selection controller mayjudge presence of the correlation 4 between the video signal of each ofthe first through n-th external peripheral pixels and the video signalof the target pixel based on the first to n-th main correlation valuesand the first through n-th sub-correlation values.

In such a noise reduction device, the first through n-th maincorrelation values and the first through n-th sub-correlation values arerespectively binarized values; the first correlation value calculatormay set the value of the k-th main correlation value to be a valueindicative of presence of a correlation with the video signal of thetarget pixel, when a value obtained by subtracting the signal value ofthe video signal of the target pixel from the signal value of the videosignal of the k-th external peripheral pixel is in a range from a firstthreshold value to a second threshold value; and the second correlationvalue calculator may set the value of the k-th sub-correlation value tobe a value indicative of presence of a correlation with the video signalof the target pixel, when a value obtained by subtracting the signalvalue of the video signal of each of the internal peripheral pixelexisting at a position symmetrical to the k-th internal peripheral pixelaround the target pixel from the signal value of the video signal of thek-th internal peripheral pixel is in a range from a third thresholdvalue to a forth threshold value.

On this occasion, with respect to each of the k-th main correlationvalue and the k-th sub-correlation value, the correlation value may be“1” which indicates there is a correlation with the video signal of thetarget pixel, or may be “0” which indicates that there is no correlationwith the video signal of the target pixel.

It is preferable that the noise reduction device further includes athreshold value calculator for determining the first and secondthreshold values by use of the signal values of the video signals of theexternal peripheral pixels.

It is also preferable that the threshold value calculator determines thefirst and second threshold values by use of an average value of thesignal values of the video signals of the external peripheral pixels.

The main correlation value indicating presence of the correlation withthe video signal of the target pixel may be defined as a correlated maincorrelation value, and the sub-correlation value indicating presence ofthe correlation with the video signal of the target pixel may be definedas a correlated sub-correlation value; the selection controller maycompare the total number of the correlated main correlation values witha first predetermined number, and the selection controller may judge thenumber of the video signals of the external peripheral pixels having thecorrelation with the video signal of the target pixel to be small whenthe total number of the correlated main correlation values is smallerthan the first predetermined number; and the selection controller mayjudge the number of the video signals of the external peripheral pixelshaving the correlations with the video signal of the target pixel to belarge when the total number of the correlated main correlation value isthe first predetermined value or more.

On this occasion, with respect to each of the k-th main correlationvalue and the k-th sub-correlation value, the value may be “1” whichindicates that there is the correlation with the video signal of thetarget pixel or may be “0” which indicates that there is no correlationwith the video signal of the target pixel.

The selection controller may compare the total number of the correlatedmain correlation values with a second predetermined number larger thanthe first predetermined number when the total number of the correlatedmain correlation values is the first predetermined number or more and:if the selection controller judges that the total number of thecorrelated main correlation values is in a range from the firstpredetermined number to the second predetermined number, the selectormay select a weighted average value of the video signals of the externalperipheral pixels corresponding to the correlated main correlationvalues calculated by the signal calculator; and if the selectioncontroller judges that the total number of the correlated maincorrelated values is larger than the second predetermined number, theselector may select a weighted average value of the video signals of theexternal peripheral pixels corresponding to both of the correlated maincorrelation values and the correlated sub-correlated values, which havebeen calculated by the signal calculator.

On this occasion, in the signal calculator, a weighted average value ofthe video signals of the external peripheral pixels corresponding to thecorrelated main correlation values is obtained in such a manner thatvalues, which are obtained by respectively multiplying the first to n-thmain correlation values with the signal values of the video signals ofthe first to n-th external peripheral pixels, are integrated, and thenthe integrated value is divided by a value obtained by integrating thefirst to n-th main correlation values. In addition, in the signalcalculator, a weighted average value of the video signals of theexternal peripheral pixels is obtained in such a manner that values,which are obtained by respectively multiplying the first to n-th maincorrelation values and the first to n-th sub-correlation values with thesignal values of the video signals of the first to n-th externalperipheral pixels, are integrated, and then the integrated value isdivided by a value which is obtained by respectively multiplying thefirst to n-th main correlation values with the first to n-thsub-correlation values.

When it is judged that the number of the video signals of the externalperipheral pixels having the correlation with the video signal of thetarget pixel is small, the selector selects the signal value of thevideo signal of the target pixel, which has been read out by the firstsignal reading-out unit.

Further, when the selection controller judges that the number of theexternal peripheral pixels having correlation with the video signal ofthe target pixel is small, the selection controller may compare thesignal value of the video signal of the target pixel, which has beenread out by the first signal reading-out unit, with a firstpredetermined value and a second predetermined value, and: if theselection controller judges that the signal value of the video signal ofthe target pixel is in either of a range smaller than the firstpredetermined value and a range larger than the second predeterminedvalue, the selector may select a weighted average value of the videosignals of the external peripheral pixels corresponding to thecorrelated sub-correlation value calculated by the signal calculator;and if the selection controller judges that the signal value of thevideo signal of the target pixel is in a range from the firstpredetermined value to a second predetermined value, the selector mayselect the signal value of the video signal of the target pixel whichhas been read out by the first signal reading-out unit.

On this occasion, with respect to each of the k-th main correlationvalue and the k-th sub-correlation value, the value may be “1”indicating that there is a correlation with the video signal of thetarget pixel, or may be “0” indicating that there is no correlation withthe video signal of the target pixel. In the signal calculator, aweighted average value of the video signals of the external peripheralpixels corresponding to the correlated main correlation values isobtained in such a manner that values, which are obtained by multiplyingthe first to n-th sub-correlation values with the respective signalvalues of the video signals of the first to n-th external peripheralpixels, are integrated, and then the integrated value is divided by avalue obtained by integrating the first to n-th main correlation values.

It is preferable that the noise reduction device further includes apredetermined value calculator for determining the first and secondpredetermined values by use of the signal values of the video signals ofthe external peripheral pixels.

It is preferable that the predetermined value calculator determines thefirst and second predetermined values by use of a value which indicatesa magnitude of variations of the signal values of the video signals ofthe external peripheral pixels.

A second aspect of the present invention is to provide a noise reductiondevice of the present invention which includes, in the noise reductiondevice reducing a noise superimposed on a video signal including pluralkinds of color signals, each of which is output by a pixel: based on arelation between a signal value of the video signal of the target pixeland a signal value of the video signal of each of external peripheralpixels arranged at positions close to the target pixel among pixelsoutputting the same color signals as that of the target pixel for whicha noise reduction is performed, a selection controller for judgingpresence of a correlation between the video signal of the target pixeland the video signal of each of the external peripheral pixels; and aselector for selecting a signal value output as the signal value of thevideo signal of the target pixel from a signal value calculated by useof the signal value of the video signal of each of the externalperipheral pixels and the signal value of the video signal of the targetpixel, in accordance with a result judged by the selection controller.

The selection controller may judge presence of a correlation between thevideo signal of the target pixel and the video signal of each of theexternal peripheral pixels based on a relation between the signal valueof the video signal of the target pixel and the signal value of thevideo signal of each of the external peripheral pixels and based on arelation between signal values of video signals of internal peripheralpixels arranged at positions adjacent to the target pixel.

The noise reduction device may further include an external defectjudgment unit for judging whether a defect has occurred in each of theexternal peripheral pixels, and the external peripheral pixels used forreducing the noise of the target pixel may be configured to be suchexternal peripheral pixels that have been judged by the external defectjudgment unit to be those where the defect has not occurred.

The external defect judgment unit may be configured to judge that thedefect has not occurred when the signal value of the video signal ofeach of the external peripheral pixels is in a range from a third to afourth predetermined value.

Incidentally, the third predetermined value may be the same value as thefirst predetermined value, and the fourth predetermined values may bethe same value as the second predetermined value.

A third aspect of the present invention is to provide an image pickupapparatus of the present invention which includes a solid state imagepickup device having plural kinds of color filters provided on surfacesof pixels and outputting video signals made to be plural kinds of colorsignals; and any of the foregoing noise reduction devices, to which thevideo signals from the solid state image pickup device are input.

In such an image pickup apparatus, an arrangement of the color filtersmay be tetragonal, or may be honeycomb. Furthermore, the color filtermay be a primary-color-based color filter.

A fourth aspect of the present invention is to provide a noise reductionmethod of the present invention which includes, in a noise reductionmethod of reducing a noise superimposed on a video signal includingplural kinds of color signals, each of which is output by a pixel,: astep of reading out a video signal of a target pixel for which a noisereduction is performed; a step of reading out video signals of externalperipheral pixels arranged around internal peripheral pixels arranged atpositions closest to the target pixel, the external peripheral pixelsbeing arranged at positions closest to the target pixel in radialdirections from the target pixel and outputting the same color signalsas the video signal of the target pixel; a step of judging presence of acorrelation between the video signal of the target pixel and a videosignal of each external peripheral pixel based on a relation between asignal value of the video signal of the target pixel and a signal valueof the video signal of each of the external peripheral pixels; a step ofcalculating the signal value of the video signal of the target pixel byuse of the signal values of the video signals of the external peripheralpixels; and a step of selecting and outputting the signal value of thevideo signal of the target pixel calculated by use of the signal valuesof the video signals of the external peripheral pixels when it is judgedthat the number of the video signals of the external peripheral pixelshaving correlations with the video signal of the target pixel is large.

Furthermore, a noise reduction method may be configured to include, in anoise reduction method of reducing a noise superimposed on a videosignal including plural kinds of color signals, each of which is outputby a pixel: a step of judging presence of a correlation between a videosignal of a target pixel and a video signal of each of the externalperipheral pixels based on a relation between a signal value of thevideo signal of the target pixel and a signal value of the video signalof each of external peripheral pixels arranged at positions in theproximity of the target pixel among pixels outputting the same colorsignals as that of the target pixel for which a noise reduction isperformed; and a step of selecting a signal value output as the signalvalue of the video signal of the target pixel from a signal valuecalculated by use of the signal values of the video signals of theexternal peripheral pixels and a signal value of the video signal of thetarget pixel, in accordance with a judged result.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an internal constitution of an imagepickup apparatus of an embodiment according to the present invention;

FIG. 2 is a diagram showing an arrangement of filters arranged in asolid-state image pickup device provided in the image pickup apparatusof FIG. 1;

FIG. 3 is a block diagram showing an internal constitution of a noisereduction circuit provided in the image pickup apparatus of FIG. 1;

FIG. 4 is a diagram showing an arrangement relation among a target pixeland peripheral pixels in the solid-state image pickup device of theimage pickup apparatus in FIG. 1;

FIGS. 5A and 5B are diagrams showing a relation between a signal valueand a threshold value at the time when a correlation value is obtainedby a correlation value calculator;

FIG. 6 is a diagram showing a relation between a signal value and athreshold value at the time when a white defect and a black defect arejudged by a defect judgment unit;

FIG. 7 is a flowchart showing an operation of a selection controller ofthe noise reduction circuit in FIG. 3;

FIG. 8 is another block diagram showing an internal constitution of thenoise reduction circuit provided in the image pickup apparatus of FIG.1; and

FIG. 9 is a diagram showing another arrangement of the filters arrangedin the solid-state image pickup device provided in the image pickupapparatus of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

An embodiment of the present invention will be described with referenceto the drawings. FIG. 1 is a block diagram showing an internalconfiguration of an image pickup apparatus having a noise reductioncircuit in this embodiment.

The image pickup apparatus of FIG. 1 includes a solid-state image pickupdevice 1 having a plurality of pixels, each outputting a video signalwhich is an electrical signal in accordance with a quantity of incidentlight of an object; an A/D conversion circuit 2 for converting the videsignal obtained by the solid-state image pickup device 1 to a digitalsignal; a noise reduction circuit 3 for performing a noise reduction forthe video signal which is converted to the digital signal by the A/Dconversion circuit 2; and a signal processing circuit 4 for performing asignal processing such as a white balance processing and a color balanceprocessing for the video signal in which the noise is reduced by thenoise reduction circuit 3. Note that the video signal is a signalincluding a color signal output from each pixel or a plurality of colorsignals output from the plurality of pixels.

In the image pickup apparatus having such a configuration, the imagepickup device 1 is a solid-state image pickup device in which pluralkinds of color filters having different light transmittances areprovided on a surface of each pixel, that is, a single-plate solid-stateimage pickup device. In terms of color filters, different kinds of colorfilters are provided for the adjacent pixels. As an array type of colorfilters provided for such a solid-state image pickup device 1, there isa Bayer pattern filter array composed of an R (Red) filter, a G (Green)filter and a B (Blue) filter as shown in FIG. 2. The Bayer patternfilter array shown in FIG. 2 has a configuration such that a line inwhich G filters and R filters are alternately aligned in a horizontaldirection and a line in which B filters and G filters are alternatelyaligned in a horizontal direction are alternately arranged in a verticaldirection. In other words, a line in which G filters and B filters arealternately aligned in a vertical direction and a line in which Rfilters and G filters are alternately aligned in a vertical directionare alternately arranged in a horizontal direction.

In the solid-state image pickup device 1 having such a Bayer patternfilter array, a video signal which is any one of R, G and B signals isoutput from the pixel having each one of R, G and B filters. The videosignal which is any one of the R, G and B signals is converted to adigital signal by the A/D conversion circuit 2, and supplied to thenoise reduction circuit 3. Presence of a correlation between a targetpixel and each of the peripheral pixels is judged, and a correctionprocess is performed by use of signal values of color signals output bythe peripheral pixels having a correlation with the target pixel,whereby noise is reduced. The noise reduction processing is performedfor each pixel in such a manner as described above, so that the noisesuperimposed on the video signals constituted by the R, G and B signalsis reduced.

The video signal from which the noise is removed by the noise reductioncircuit 3 is supplied to the signal processing circuit 4. In the signalprocessing circuit 4, a signal processing such as a white balanceprocessing, a color balance processing and an interpolation processingis performed for each of the R, G and B signals constituting the videosignals. Then, the video signal obtained by performing the predeterminedsignal processing in the signal processing circuit 4 is output. Thevideo signal output is recorded onto a recording medium, or reproducedby a display device.

Descriptions of the noise reduction circuit 3 in the image pickupapparatus, which operates as described above, will be made. FIG. 3 is ablock diagram showing an internal configuration of the noise reductioncircuit provided in the image pickup apparatus of FIG. 1. As shown inFIG. 3, the noise reduction circuit 3 includes a memory 31 for storingthe video signal converted to the digital signal by the A/D conversioncircuit 2; a signal reading-out unit 32 a for reading out the videosignal of the target pixel, for which a noise reduction processing is tobe performed, from the memory 31; a signal reading-out unit 32 b forreading-out a video signal of each of peripheral pixels from the memory31, which is positioned at a position separate from the target pixelwith one pixels interposed there between and provided with the same kindof color filter; and a signal reading-out unit 32 c for reading out avideo signal of each of peripheral pixels from the memory 31, which ispositioned one pixel away from the target pixel.

Furthermore, the noise reduction circuit 3 includes a differencecalculator 33 a for obtaining a difference between a signal value of thevideo signal of the target pixel and a signal value of the video signalof each of the peripheral pixels, which have been respectively read outby the signal reading-out units 32 a and 32 b; a difference calculator33 b for obtaining a difference between two signal values among thesignal values of the video signals of the peripheral pixels which havebeen read out by the signal reading-out unit 32 c; a correlation valuecalculator 34 a for comparing the difference obtained by the differencecalculator 33 a with a threshold value so as to obtain a correlationvalue of each of the peripheral pixels; a correlation value calculator34 b for comparing the difference obtained by the difference calculator33 b with a threshold value to a correlation value of each of theperipheral pixels; a correlation value addition unit 35 a for obtaininga total sum of the correlation values obtained by the correlation valuecalculator 34 a; a correlation value addition unit 35 b for obtaining atotal sum of the correlation values obtained by the correlation valuecalculator 34 b; a correlation value multiplication unit 35 c formultiplying the correlation values obtained respectively by thecorrelation value calculation units 34 a and 34 b; a correlation valueaddition unit 35 d for obtaining a total sum of the correlation valuesmultiplied by the correlation value multiplication unit 35 c; and adefect judgment unit 36 for judging whether the video signal read out bythe signal reading-out unit 32 a is a white defect or a black defect.

Furthermore, the noise reduction circuit 3 includes an average valuecalculator 37 a for obtaining a weighted average value of the signalvalues of the peripheral pixels which have been judged by thecorrelation value calculator 34 a to have correlations with the targetpixel; an average value calculator 37 b for obtaining a weighted averagevalue of the signal values of the peripheral pixels which have beenjudged by the correlation value calculator 34 b to have correlationswith each other; an average value calculator 37 c for obtaining aweighted average value of the signal values of the peripheral pixelswhich have been judged by the correlation value calculation units 34 aand 34 b to have the correlations with the target pixel; a selectioncontroller 38 for controlling a selector 39 to be described later, basedon a calculation result of the correlation value addition unit 35 a anda judgment result of the defect judgment unit 36; and the selector 39,which is controlled by the selection controller 38 and selects any oneof the signal value of the video signal of the target pixel and thesignal values obtained by the average value calculators 37 a to 37 c tooutput the selected signal value as the signal value of the video signalof the target pixel. Herein, the weighted average value is an averagevalue obtained for a plurality of values while the degree ofconsideration for each of these values is made different. In thisembodiment, all of the weights in the cases where the weighted averagevalues are calculated shall be one. Specifically, each of the pluralityof signal values for which the average value is obtained is consideredevenly.

An operation of the noise reduction circuit 3 configured as describedabove will be described with reference to the drawings. FIG. 4 is adiagram showing an arrangement relation between the target pixel andeach of the peripheral pixels in the solid-state image pickup device 1.FIGS. 5A and 5B are diagrams, each showing a relation between adifference of the signal values and a threshold value at the time when acorrelation value is obtained by the correlation value calculation units34 a and 34 b. FIG. 6 is a diagram showing a relation between the signalvalue and a predetermined value at the time when a white defect and ablack defect are judged by the defect judgment unit 36. FIG. 7 is aflowchart showing an operation of a selection controller 38.

As shown in FIG. 4, when five by five pixels with five lines in ahorizontal direction and with five lines in a vertical direction aredenoted as G00 to G44, and when a pixel G22 positioned at the center ofthe arrangement of FIG. 4 is the target pixel, the video signal of thepixel G22 is read out by the signal reading-out unit 32 a. Specifically,the signal reading-out unit 32 a is a first signal reading-out unit forreading out the video signal of the target pixel, for which the noisereduction is performed. Furthermore, the video signals output by thepixels G02, G42, G20 and G24, which are arranged respectively atpositions one pixel away from the target pixel G22 in either of thehorizontal and vertical directions, are read out as the video signals ofthe peripheral pixels by the signal reading-out unit 32 b. The videosignals output by the pixels G00, G44, G04 and G40, which are arrangedrespectively at positions one pixel away from the target pixel G22 indirections slanted from either of the horizontal and vertical directionsby 45 degrees (hereinafter referred to as diagonal directions), are alsoread out as the video signals of the peripheral pixels as the signalreading-out unit 32 b.

The video signals of the peripheral pixels G00, G02, G04, G20, G24, G40,G42 and G44 are R signals when the video signal of the target pixel G22is an R signal. The video signals thereof are G signals when the videosignal of the target pixel G22 is a G signal. The video signals thereofare B signals when the video signal of the target pixel G22 is a Bsignal. In other words, the peripheral pixels G00, G02, G04, G20, G24,G40, G42 and G44 are pixels which output the same color signals as thatof the target pixel G22. In addition, the peripheral pixels G00, G02,G04, G20, G24, G40, G42 and G44 are pixels arranged at the positions inthe proximity of the target pixel G22. Furthermore, in the followingdescriptions, the peripheral pixels G00, G02, G04, G20, G24, G40, G42and G44 are arranged at the positions one pixel away from the targetpixel G22, are called “external peripheral pixels”. Specifically, theperipheral pixels G00, G02, G04, G20, G24, G40, G42 and G44 are arrangedaround internal peripheral pixels arranged at positions closest to thetarget pixel G22, and are arranged at the positions closest to thetarget pixel G22 in radial directions from the target pixel G22, theperipheral pixels G00, G02, G04, G20, G24, G40, G42 and G44 outputtingthe same color signal as that of the target pixel G22. In addition, thesignal reading-out unit 32 b is a second signal reading-out unit forreading out the video signals of the external peripheral pixels. Notethat in this embodiment, the number n of the external peripheral pixelsis 8, and that the pixels G00, G02, G04, G20, G24, G40, G42 and G44 arethe first to n-th external peripheral pixels, sequentially.

Furthermore, the video signals of the pixels G12, G32, G21 and G23,which are arranged at the positions adjacent to the target pixel G22 ineither of the horizontal and vertical directions, and the video signalsof the pixels G11, G33, G13 and G31, which are arranged at the positionsadjacent to the target pixel G22 in the two diagonal directions, areread out by the signal reading-out unit 32 c as the video signals of theperipheral pixels. Accordingly, when the video signal of the targetpixel G22 is the R signal, the video signals of the peripheral pixelsG11, G13, G31 and G33 are B signals, and the video signals of the G12,G21, G23 and G32 is G signals. When the video signal of the target pixelG22 is the B signal, the video signals of the G11, G13, G31 and G33 areG signals.

Furthermore, when the video signal of the target pixel G22 is the Gsignal, the video signals of the peripheral pixels G11, G13, G31 and G33are a G signal, and the video signals of the G12, G21, G23 and G32 areeither an R signal or a B signal. At this time, if the video signals ofthe peripheral pixels G12 and G32 are R signal, the video signals of theG21 and G23 are the B signal and if the video signals of the peripheralpixels G12 and G32 are the B signal, the video signals of the G21 andG23 are the R signal. As described above, the two peripheral pixelsarranged so as to interpose the target pixel there between on the samestraight line output the same color signal. In addition, in thefollowing descriptions, the peripheral pixels G11, G12, G13, G21, G23,G32 and G33, which are arranged at positions next to the target pixelG22, that is, at positions closest (adjacent) to the target pixel G22 inradial directions from the target pixel G22, are called “internalperipheral pixels”. In addition, the signal reading-out unit 32 c is athird signal reading-out unit for reading out the video signals of theinternal peripheral pixels. In this embodiment, the number n of theinternal peripheral pixels also is 8, and the pixels G11, G12, G13, G21,G23, G31, G32 and G33 are the first to the n-th internal peripheralpixels, sequentially.

Accordingly, a direction extending from the k-th (k: a natural numbersatisfying 1≦k≦8) external peripheral pixel to the target pixel and adirection extending from the k-th internal peripheral pixel to thetarget pixel are the same.

As described above, the video signal of the target pixel is read out bythe signal reading-out unit 32 a, and the video signals of the eightexternal peripheral pixels are read out by the signal reading-out unit32 b. The video signals of the eight internal peripheral pixels are readout by the signal reading-out unit 32 c. Then, the signal value of thevideo signal of the target pixel, which has been read out by the signalreading-out unit 32 a, and the signal values of the video signals of theexternal peripheral pixels read out by the signal reading-out unit 32 bare supplied to the difference calculator 33 a. In this differencecalculator 33 a, the signal value of the video signal of the targetpixel is subtracted from the respective signal values of the videosignals of the external peripheral pixels, and eight difference values(hereinafter referred to as “first difference values”) are obtained.Each of the first difference values thus obtained is a value indicatinga relation between the target pixel and each of the external peripheralpixels.

Specifically, when the signal values of the video signals in therespective pixels G00 through G44 are denoted as g00 through g44, thefirst difference values mD00, mD02, mD04, mD20, mD24, mD40 and mD44 areobtained in the following manners.

-   -   mD00=g00−g22    -   mD02=g02−g22    -   mD04=g04−g22    -   mD20=g20−g22    -   mD24=g24−g22    -   mD40=g40−g22    -   mD42=g42−g22    -   mD44=g44−g22

Furthermore, the signal values of the video signals of the internalperipheral pixels read out by the signal reading-out unit 32 c aresupplied to the difference calculator 33 b. In this differencecalculator 33 b, eight difference values (hereinafter referred to as“second difference values”) are obtained by performing subtractionsbetween the signal values of the video signals of the two internalperipheral pixels existing at positions symmetrical to each other whenthe target pixel is taken as their center. Specifically, the seconddifference value is a result obtained by subtracting, from the signalvalue of the video signal of one of the n peripheral pixels, the signalvalue of the video signal of the other pixel, among the first to n-thinternal peripheral pixels, existing at a position symmetrical to theone of the n peripheral pixels when the target pixel is taken as theircenter.

At this time, by use of the two internal peripheral pixels arranged onthe same straight line, a difference value by subtracting one internalperipheral pixel from the other internal peripheral pixel and adifference value by subtracting the other internal peripheral pixel fromone internal pixel are obtained. The second difference value obtained inthe above described manner is a value indicating a correlation betweenthe target pixel and the external peripheral pixel whose positionalrelation with the target pixel is the same as the positional relationbetween the two internal peripheral pixels used to obtain the seconddifference value.

Specifically, the second difference values sD00, sD02, sD04, sD20, sD24,sD40, sD42 and sD44 are obtained in the following manner.

-   -   sD00=g11−g33    -   sD02=g12−g32    -   sD04=g13−g31    -   sD20=g21−g23    -   sD24=g23−g21    -   sD40=g31−g13    -   sD42=g32−g12    -   sD44=g33−g11

Subsequently, the first difference values mDi (i represents 00, 02, 04,20, 24, 40, 42 and 44), which have been obtained by the differencecalculator 33 a, are respectively supplied to the correlation valuecalculator 34 a. The correlation value calculator 34 a compares thefirst difference values mDi with threshold values −mth1 and mth2(−1023<−mth1<0<mth2<1023). Note that “−mth1” indicates a first thresholdvalue, and that “mth2” indicates a second threshold value. Based on theabove comparison results, correlation values mCi (hereinafter referredto as “main correlation values mCi) are obtained, which is set to “1”for the peripheral pixels Gi judged to have correlations with the targetpixel G22, and which is set to “0” for the peripheral pixels Gi judgedto have no correlations with the target pixel G22. Note that in the A/Dconversion circuit 2 of this embodiment, the video signal shall beoutput as a 10-bit digital signal (video signal having a signal value of0 through 1023).

As shown in FIG. 5A, when the first difference value mDi is within therange expressed by −mth1≦mDi≦mth2, this correlation value calculator 34a judges that there is a correlation between the target pixel G22 andthe peripheral pixel Gi, and sets the main correlation value mCi to “1”.When the first difference value mDi is in the range expressed either bymDi<−mth1 or by mDi>mth2, the correlation value calculator 34 a judgesthat there is no correlation between the target pixel G22 and theperipheral pixel Gi, and sets the main correlation value mCi to “0”.

Note that the main correlation values mCi (“i” indicates 00, 02, 04, 20,24, 40, 42 and 44) are first to n-th main correlation values indicatingthe presence of correlations between the respective video signals of thefirst to n-th external peripheral pixels and the video signal of thetarget pixel. In addition, note that the correlation value calculator 34a is a first correlation value calculator for obtaining first to n-thmain correlation values.

The second difference values sDi obtained by the difference calculator33 b are respectively supplied to the correlation value calculator 34 b.As shown in FIG. 5B, the correlation value calculator 34 b compares thesecond difference values sDi with threshold values −sth1, sth2(−1023<−sth1<0<sth2<1023). Based on the comparison results, correlationvalues sCi (hereinafter referred to as “sub-correlation values sCi”) areobtained, which is set to “1” for the peripheral pixels Gi judged tohave a correlation with the target pixel G22, and which is set to “0”for the peripheral pixels Gi judged to have no correlation with thetarget pixel G22.

As shown in FIG. 5B, when the second difference value sDi is within therange expressed by −sth1≦sDi≦sth2, the correlation value calculator 34 bjudges that there is a correlation between the target pixel G22 and theperipheral pixel Gi, and sets the sub-correlation value sCi to “1”. Inaddition, when the second difference value sDi is in the range expressedeither by sDi<−sth1 or by sDi>sth2, the correlation value calculator 34b judges that there is no correlation between the target pixel G22 andthe peripheral pixel Gi, and sets the sub-correlation value sCi to “0”.

Note that the sub-correlation values sCi (“i” indicates 00, 02, 04, 20,24, 40, 42 and 44) are first to n-th sub-correlation values indicatingthe presence of correlations between the respective video signals of thefirst to n-th external peripheral pixels and the video signal of thetarget pixel. In addition, note that the correlation value calculator 34b is a second correlation value calculator for obtaining first to n-thsub-correlation values.

Then, the main correlation values mCi obtained by the correlation valuecalculator 34 a are respectively supplied to the correlation valueaddition unit 35 a, and the sub-correlation values sCi obtained by thecorrelation value calculator 34 b are respectively supplied to thecorrelation value addition unit 35 b. In the correlation value additionunit 35 a, the main correlation values mCi are added up, and a total sumΣmC is obtained. In the correlation value addition unit 35 b, thesub-correlation values sCi are added up, and a total sum ΣsC isobtained. Specifically, the total sum ΣmC obtained by the correlationvalue addition unit 35 a and the total sum ΣsC obtained by thecorrelation value addition unit 35 b are obtained in the followingmanner.

-   -   ΣmC=mC00+mC02+mC04+mC20+mC24+mC40+mC42+mC44    -   ΣsC=sC00+sC02+sC04+sC20+sC24+sC40+sC42+sC44

Furthermore, the main correlation values mCi and the sub-correlationvalues sCi, which have been obtained by the respective correlation valuecalculators 34 a and 34 b, are supplied to the correlation valuemultiplication unit 35 c. The correlation value multiplication unit 35 cmultiplies the main correlation values mCi and the sub-correlationvalues sCi, respectively. Then, the correlation value addition unit 35 dadds up the values obtained by the above multiplications, thus obtaininga total sum Σ(mC*sC). Specifically, the total sum Σ(mC*sC) obtained bythe correlation value multiplication unit 35 c and the correlation valueaddition unit 35 d is obtained in the following manner.

-   -   Σ(mC*sC)=mC00*sC00+mC02*sC02+mC04*sC04+mC20*sC20+mC24*sC24+mC40*sC40+mC42*sC42+mC44*sC44        Note that the main correlation value and the sub-correlation        value, which are multiplied with each other, are the k-th main        correlation value and the k-th sub-correlation value by the        same k. Accordingly, the main correlation value and the        sub-correlation value, which are multiplied with each other, are        correlation values respectively relating to the external        peripheral pixels and the internal peripheral pixels in the same        direction with respect to the target pixel.

Furthermore, the video signal read out by the signal reading-out unit 32a is supplied to the defect judgment unit 36. The defect judgment unit36 compares the signal value of the video signal of the target pixelwith each of predetermined values Lm and LM (0<Lm<LM<1023), as shown inFIG. 6. Specifically, the defect judgment unit 36 confirms whether thesignal value g22 of the video signal of the target pixel G22 is in therange expressed by Lm<g22<LM. Note that Lm is a first predeterminedvalue and LM is a second predetermined value. Then, when the signalvalue g22 is in the range expressed by g22<Lm, the defect judgment unit36 judges that a black defect occurred in the target pixel G22. When thesignal value g22 is in the range expressed by g22>LM, the defectjudgment unit 36 judges that a white defect occurred in the target pixelG22. In addition, when the signal value g22 is in the range expressed byLm<g22<LM, the defect judgment unit 36 judges that there is no defect inthe target pixel G22. Note that the black defect and the white defectare signals which are generated by a structural malfunction and the likeof pixels constituting the solid-state image pickup device 1.

In the above described manner, respectively in the correlation valueaddition units 35 a and 35 b, the total sums of the correlation valuesrespectively of the external peripheral pixels and of the internalperipheral pixels are obtained. In addition, the total sum of theintegrated values of the correlation values of the respective externalperipheral pixels and the respective internal peripheral pixels areobtained in the correlation value addition unit 35 d. Furthermore, inthe defect judgment unit 36, the presence of the defect in the targetpixel is judged. At this time, the average value calculators 37 a to 37c carry out operations in the following manner.

First, the signal values gi of the video signals of the externalperipheral pixels Gi read out by the signal reading-out unit 32 b, thecorrelation values mCi obtained by the correlation value calculator 34a, and the total sum ΣmC obtained by the correlation value addition unit35 a are respectively supplied to the average value calculator 37 a.Subsequently, in the average value calculator 37 a, a weighted averagevalue g_(av)1 of the signal values of the video signals havingcorrelations with the target pixel is obtained. Specifically, a valueobtained by adding the signal values gi having the correlation valuesmCi of “1” is divided by the total sum ΣmC, thus the weighted averagevalue g_(av)1 is obtained.

-   -   g_(av)1=(g00*mC00+g02*mC02+g04*mC04+g20*mC20+g24*mC24+g40*mC40+g42*mC42+g44*mC44)/ΣmC

In addition, the signal values gi of the video signals of the externalperipheral pixels Gi read out by the signal reading-out unit 32 b, thecorrelation values sCi obtained by the correlation value calculator 34b, and the total sum ΣsC obtained by the correlation value addition unit35 b are respectively supplied to the average value calculator 37 b.Subsequently, in the average value calculator 37 b, a weighted averagevalue g_(av)2 of the signal values of the video signals havingcorrelations with the target pixel is obtained. Specifically, a valueobtained by adding the signal values gi having the correlation valuessCi of “1” is divided by the total sum ΣsC, thus the weighted averagevalue g_(av)2 is obtained.

-   -   g_(av)2=(g00*sC00+g02*mC02+s04*sC04+g20*sC20+g24*sC24+g40*sC40+g42*sC42+g44*sC44)/ΣsC

Furthermore, the signal values gi of the video signals of the externalperipheral pixels Gi read out by the signal reading-out unit 32 b, thecorrelation values mCi and sCi obtained by the correlation valuecalculators 34 a and 34 b, and the total sum Σ(mC*sC) obtained by thecorrelation value addition unit 35 d are respectively supplied to theaverage value calculator 37 c. Subsequently, in the average valuecalculator 37 c, a weighted average value g_(av)3 of the signal valuesof the video signals having correlations with the target pixel isobtained. Specifically, a value obtained by adding the signal values gihaving the correlation values mCi*sCi of “1” is divided by the total sumΣ(mC*sC), thus the weighted average value g_(av)3 is obtained.

-   -   g_(av)3=(g00*mC00*sC00+g02*mC02*sC02+g04*mC04*sC04+g20*mC20*sC20+g24*mC24*sC24+g40*mC40*sC40+g42*mC42*sC40+g44*mC44*sC44)/Σ(mC*sC)

Each of the units carries out an operation in the above describedmanner. The selection controller 38 performs an operation to judgepresence of a correlation between the video signals of the target pixeland of each of the external peripheral pixels in accordance with theflowchart of FIG. 7, whereby the selector 39 is controlled. First, theselection controller 38 judges whether the total sum ΣmC from thecorrelation value addition unit 35 a is n1 or less (S1). Herein, n1 is afirst predetermined number compared with the total number of the maincorrelation value mC having a value of “1”. Specifically, in S1, it isjudged whether the number of the external peripheral pixels havingcorrelations with the target pixel G22 is n1 or less, and when thenumber of the pixels is judged to be larger than n1, the selectioncontroller 38 judges that there is a correlation with the externalperipheral pixels. Then, when the total sum ΣmC is larger than n1 (No),it is judged whether the total sum ΣmC is n2 or less (S2). Herein, n2 isa second predetermined value larger than the first predetermined valuen1.

In S2, it is judged whether the number of the external peripheral pixelshaving correlations with the target pixel G22 is n2 or less, and whenthe number of the pixels is n2 or less (YES), the selection controller38 judges, by use of the correlations with the external peripheralpixels, a direction where the correlation is stronger. In addition, whenthe number of the pixels is larger than n2 (NO), the selectioncontroller 38 judges, by use of the respective correlations of theexternal and internal peripheral pixels, a direction where thecorrelation is stronger. Specifically, when the number of the externalperipheral pixels having the correlation with the target pixel G22 is n2or more, the selection controller 38 confirms the external peripheralpixels having strong correlations with the video signal of the targetpixel, based on the first to n-th main correlation values and the firstto n-th sub-correlation values.

Accordingly, when the total sum ΣmC is n2 or less (YES), the weightedaverage value g_(av)1 of the external peripheral pixels, which has beenobtained by the average value calculator 37 a, is selected by theselector 39, and is output as the signal value of the video signal ofthe target pixel G22 (S3). In addition, when the total sum ΣmC is largerthan n2 (NO), the weighted average value g_(av)3 of the externalperipheral pixels, which has been obtained by the average valuecalculator 37 c, is selected by the selector 39, and is output as thesignal value of the video signal of the target pixel G22 (S4).

Furthermore, when the total sum ΣmC is n1 or less in S1 (YES), thedefect judgment unit 36 judges, based on the judgment result of thedefect judgment unit 36, whether either a white defect or a black defectexists in the target pixel G22 (S5). Then, when it is judged that eithera white defect or a black defect exists in the target pixel G22 (YES),the selection controller 38 judges, by use of the correlation of theinternal peripheral pixel, a direction where the correlation isstronger. Accordingly, the weighted average value g_(av)2 of theinternal peripheral pixels obtained by the average value calculator 37 bis selected by the selector 39, and output as the signal value of thevideo signal of the target pixel G22 (S6). In addition, when it isjudged that either of the white and black defects does not exist in thetarget pixel G22 (NO), it is judged that the signal value of the videosignal of the target pixel G22 is an independent value which has nocorrelation with the external and internal peripheral pixels.Accordingly, the signal value g22 of the video signal of the targetpixel G22, which has been read out by the signal reading-out unit 32 a,is selected by the selector 39, and output as the signal value of thevideo signal of the target pixel G22 (S7).

The video signal of the target pixel, which has been thus selected bythe selector 39, is output to the signal processing circuit 4 in thesubsequent stage as a video signal in which noises are reduced. Thus,the video signal is subjected to various kinds of signal processing inthe signal processing circuit 4.

Specifically, in accordance with the result judged by the selectioncontroller 38, any one of the average value calculators 37 a to 37 cfunctions as a signal calculation unit for calculating the signal valueof the video signal of the target pixel by use of the signal values ofthe video signals of the external peripheral pixels. In addition, inaccordance with the result confirmed by the selection controller 38, theselector 39 selects a signal value output as the signal value of thevideo signal of the target pixel from among the g_(av)1, g_(av)2,g_(av)3, and the signal value of the video signal of the target pixelG22, where the g_(av)1, g_(av)2 and g_(av)3 are the signal valuescalculated by use of the signal values of the video signals of theexternal peripheral pixels.

(Effects)

Each of the external peripheral pixels is one pixel away from the targetpixel, and outputs the same color signal as the target pixel. Inaddition, based on the relation between the signal value of the videosignal of each of the external peripheral pixels, which is the samecolor signal as the target pixel, and the signal value of the videosignal of the target pixel, the selection controller 38 judges whetherthere is the correlation between the external peripheral pixel and thetarget pixel. Therefore, the correlation between the external peripheralpixel and the target pixel can be comprehended accurately. Accordingly,it is possible to perform the noise reduction processing moreaccurately.

Furthermore, since the external peripheral pixels are arranged at thepositions which are one pixel away from the target pixel, a degree ofcorrelation between the video signals is lower than that of the pixelsadjacent to each other. Therefore, as in the case where a pixelirrelevant to the target pixel exists between the external peripheralpixel and the target pixel, even when the correlation between theexternal peripheral pixel and the target pixel is practically low, itmay be judged that there is correlation, in the case where the signalvalue of the video signal of the external peripheral pixel and thesignal value of the video signal of the target pixel are approximatelyequal to each other accidentally. In this case, since the average valuecalculation for the signal values of the video signals of the peripheralpixels is performed whereby the noise reduction processing is performed,there is a possibility that degradation of the resolution is broughtabout. However, in this embodiment, after considering not only therelation between the external peripheral pixel and the target pixel butalso the relation between the internal peripheral pixels, presence of acorrelation between the external peripheral pixel and the target pixelis judged. Accordingly, it is possible to suppress degradation of theresolution while performing the noise reduction processing moreappropriately.

The provision of the noise reduction circuit according to thisembodiment makes it possible to realize the noise reduction device. Inaddition, the noise reduction method can be realized by use of the noisereduction circuit according to this embodiment.

In an image pickup apparatus for picking up dynamic images and staticimages, it is possible to reduce a noise more appropriately by use ofthe noise reduction circuit according to this embodiment. Particularly,the noise reduction circuit according to this embodiment can be appliedto an image pickup apparatus having a single plate solid-state imagepickup device such as a CCD and a CMOS sensor.

Second Embodiment

As shown in FIG. 8, a noise reduction circuit in a second embodimentincludes a threshold value calculator 40 and a predetermined valuecalculator 41 in addition to the internal components provided in thenoise reduction circuit in the first embodiment. Note that the sameparts as those in the first embodiment are denoted by the same referencenumerals and descriptions of them are omitted.

The threshold value calculator 40 obtains threshold values −mth1 andmth2 by use of the video signal of each of the external peripheralpixels. Furthermore, the threshold value calculator 40 obtains thethreshold values −mth1 and mth2 by use of an average value of the videosignals of the external peripheral pixels.

To be concrete, the threshold value calculator 40 acquires the signalvalues of the video signals of the external peripheral pixels from thesignal reading-out unit 32 b, and divides the total sum of the signalvalues by the number of the external peripheral pixels, thus obtainingan average value g_(av)4. Specifically, the average value g_(av)4 isobtained by the following equation.

-   -   g_(av)4=(g00+g02+g04+g20+g24+g40+g42+g44)/8

By use of a product of the obtained average value g_(av)4 with asuitable adjusting value adj1, the threshold value calculator 40 obtains−mth1 and mth2 in the following manner.

-   -   −mth1=−g_(av)4*adj1    -   mth2=g_(av)4*adj1

Note that adj1 should be set to a value which is considered to besuitable in the range of 0 to 1 after performing the noise reductionprocessing actually. adj1 is set to, for example, ⅛.

The threshold values, which have been obtained by the threshold valuecalculator 40, are supplied to the correlation value calculator 34 a,and are used for the judgment of presence of a correlation between eachof the external peripheral pixels and the target pixel.

The predetermined value calculator 41 obtains Lm and LM by use of thesignal values of the video signals of the external peripheral pixels. Inaddition, the predetermined value calculator 41 obtains Lm and LM by useof a value indicating a magnitude of variations of the video signals ofthe external peripheral pixels. The value var1 indicating the magnitudeof the variations of the signal values of the video signals of theexternal peripheral pixels can be obtained by use of the average valueg_(av)4 of the signal values of the video signals of the externalperipheral pixels obtained in the above described manner.

To be more concrete, the predetermined value calculator 41 acquires thesignal values of the video signals of the external peripheral pixelsfrom the signal reading-out unit 32 b, and obtains the average valueg_(av)4 as described above. The predetermined value calculator 41 addsthe differences between the obtained g_(av)4 and each of the signalvalues of the video signals of the external peripheral pixels, anddivides the total sum obtained in such a manner by the number of theexternal peripheral pixels. Thus, var1 is obtained.

-   -   var1=(|g00−g_(av)4|+|g02−g_(av)4|+|g04−g_(av)4|+|g20−g_(av)4|+|g24−g_(av)4|+|g40−g_(av)4|+|g42−g_(av)4|+|g44−g_(av)4|)/8    -   where |x| represents an absolute value of x

The first predetermined value Lm is obtained by subtracting a product ofthe obtained var1 and a suitable adjusting value adj2 from the averagevalue g_(av)4. In addition, the second predetermined value LM isobtained by adding a product of the obtained var1 and a suitableadjusting value adj2 to the average value g_(av)4

-   -   Lm=g_(av)4−var1*adj2    -   LM=g_(av)4+var1*adj2

Note that adj2 should be set to a value which is considered to besuitable after performing the noise reduction processing actually. adj2is set to, for example, 2.

The predetermined values Lm and LM, which have been obtained by thepredetermined value calculator 41, are supplied to the defect judgmentunit 36, and are used for judging whether a defect has occurred in thetarget pixel G22.

(Effects)

According to this embodiment, the threshold values −mth1 and mth2 usedfor the judgment of presence of a correlation by the correlation valuecalculator 34 a is obtained based on the signal values of the videosignals of the external peripheral pixels. If the threshold values areset independently from the signal values of the video signals of theexternal peripheral pixels, it may be judged that there is correlationeven when there no correlation actually in the case of an imageoutputting large signal values as a whole. On the other hand, in thecase of an image outputting small signal values as a whole, it may bejudged that there is no correlation even when there is actuallycorrelation. Therefore, by obtaining the correlation value by use of thethreshold values obtained by the average value of the signal values ofthe video signals of the external peripheral pixels, the correlationvalue calculator 34 a can judge presence of a correlation between thevideo signal of each of the external peripheral pixels and the videosignal of the target pixel more accurately. Accordingly, it is possibleto perform the noise reduction processing more accurately.

Furthermore, the first predetermined value Lm and the secondpredetermined value LM, which are used by the defect judgment unit 36,are obtained based on the signal values of the video signals of theexternal peripheral pixels. When the first predetermined value Lm andthe second predetermined value LM are set independently from the videosignals of the external peripheral pixels, the judgment that a defecthas occurred in the target pixel is apt to be made for images showinglarge signal values as a whole and showing small signal values as awhole. Therefore, the judgment for the defect is performed by use of thefirst and second predetermined values, which have been obtained by useof the average value of the signal values of the video signals of theexternal peripheral pixels, whereby the defect judgment unit 36 canjudge more accurately whether the defect has occurred in the targetpixel. Accordingly, it is possible to perform the noise reductionprocessing more accurately.

(Modifications)

The present invention is not limited to the above embodiments, andvarious modifications may be possible.

In the above described embodiments, though the average value calculators37 a to 37 c obtain the weighted average value of the signal values ofthe video signals of the external peripheral pixels having correlationswith the target pixel, the average value calculators 37 a to 37 c mayobtain the weighted average value of the signal values of the videosignals of both the target pixel and the external peripheral pixelshaving the correlation with the target pixel. Specifically, the weightedaverage values g_(av)1 through g_(av)3 obtained by the average valuecalculators 37 a to 37 c are determined in the following manner.

-   -   g_(av)1=(g00*mC00+g02*mC02+g04*mC04+g20*mC20+g24*mC24+g40*mC40+g42*mC42+g44*mC44+g22)/(ΣmC+1)    -   g_(av)2=(g00*sC00+g02*sC02+g04*sC04+g20*sC20+g24*sC24+g40*sC40+g42*sC42+g44*sC44+g22)/(ΣsC+1)    -   g_(av)3=(g00*mC00*sC00+g02*mC02*sC02+g04*mC04*sC04+g20*mC20*sC20+g24*mC24*sC24+g40*mC40*sC40+g42*mC42*sC42+g44*mC44*sC44+g22)/(Σ(mC*sC)+1)

Alternatively, in the first and second embodiments, the weighted averagevalues g_(av)1, g_(av)2 and g_(av)3 are determined by considering thesignal values of the video signals of the external peripheral pixelsevenly, which are judged to have correlations with the target pixel.Specifically, all of the weights when the weighted average values arecalculated are “1”. However, the average value calculators 37 a through37 c may calculate the weighted average values g_(av)1, g_(av)2 andg_(av)3 by changing the degree of the consideration for the respectiveexternal peripheral pixels. For example, the weighted average valuecalculators 37 a through 37 c may calculate the weighted average valuesby making the weights of the signal values of the video signals of theexternal peripheral pixels, which have large correlations with thetarget pixel, large and by making the weights of the signal values ofthe video signals of the external pixels, which have small correlationswith the target pixel, small. For example, the main correlation valuesmCi used in calculating the weighted average values g_(av)1 and g_(av)3are set to “1” by the correlation value calculator 34 a, when the firstdifference values mDi satisfy −mth1≦mDi≦mth2. Similarly, thesub-correlation values sCi used in calculating the weighted averagevalues g_(av)2 and g_(av)3 are set to “1” by the correlation valuecalculator 34 b when the second difference values sDi satisfy−sth1≦sDi≦sth2. However, the correlation value calculators 34 a and 34 bmay set the main correlation values mCi or the sub-correlation valuessCi to a value which is larger than “1”, for example, “2”, when thefirst difference values mDi or the second difference values sDi arecloser to “0”. In this case, the main correlation values mCi or thesub-correlation values sCi function as the weights. Thus, it is possibleto perform the noise reduction processing more accurately.

Furthermore, as described above, when the average value calculators 37 athrough 37 c use the signal value of the video signal of the targetpixel in calculating the average values, the average value calculators37 a through 37 c may calculate the weighted average values g_(av)1,g_(av)2 and g_(av)3 by making the weight for the signal value of thevideo signal of the target pixel large. For example, when weight for theexternal peripheral pixels having the correlations with the target pixelis set to “1” and when the weight for the target pixel is set to “2”,the weighted average values g_(av)1, g_(av)2 and g_(av)3 are calculatedin the following manner.

-   -   g_(av)1=(g00*mC00+g02*mC02+g04*mC04+g20*mC20+g24*mC24+g40*mC40+g42*mC42+g44*mC44+g22*2)/(ΣmC+2)    -   g_(av)1=(g00*sC00+g02*sC02+g04*sC04+g20*sC20+g24*sC24+g40*sC40+g42*sC42+g44*sC44+g22*2)/(ΣsC+2)    -   g_(av)3=(g00*mC00*sC00+g02*mC02*sC02+g04*mC04*sC04+g20*mC20*sC20+g24*mC24*sC24+g40*mC40*sC40+g42*mC42*sC42+g44*mC44*sC44+g22*2)/(Σ(mC*sC)+2)

Furthermore, in this embodiment, though the arrangement of the pixels inthe solid-state image pickup device is a tetragonal arrangement as shownin FIG. 2, the arrangement thereof may be a honeycomb arrangement asshown in FIG. 9, which is obtained by slanting the tetragonalarrangement by 45 degrees. Also in the case of the arrangement as shownin FIG. 9, the pixels arranged at the positions one pixel away from thetarget pixel in the horizontal and vertical directions as well as in thetwo diagonal directions are external peripheral pixels, and the pixelsarranged at the positions adjacent to the target pixel in the horizontaland vertical directions as well as the two diagonal directions areinternal peripheral pixels, whereby a noise reduction processing similarto that described above can be performed.

Furthermore, though the threshold value calculator 40 and thepredetermined value calculator 41 in the second embodiment areconfigured to determine the average value of the signal values of thevideo signals of the external peripheral pixels, they may be configuredto determine an average value of the signal values of the video signalsof the external peripheral pixels and of the target pixel. Specifically,g_(av)4 is determined in the following manner.

-   -   g_(av)4=(g00+g02+g04+g20+g24+g40+g42+g44+g22)/9

In this case, a value var1 indicating a magnitude of variations isdetermined in the following manner.

-   -   var1=(g00−g_(av)4|+|g02−g_(av)4|+|g04−g_(av)4|+|g20−g_(av)4|+|g24−g_(av)4|+|g40−g_(av)4|+|g42−g_(av)4|+|g44−g_(av)4|+|g22−g_(av)4|)/9

Furthermore, the predetermined value calculator 41 may use a standarddeviation var2 of the signal values of the video signals of the externalperipheral pixels as a value indicating a magnitude of the variations.var2 is determined in the following manner.

-   -   var2={(|g00−g_(av)4|²+|g02−g_(av)4|²+|g04−g_(av)4|²+g20−g_(av)4|²+|g24−g_(av)4|²+|g40−g_(av)4|²+|g42−g_(av)4|²+|g44−g_(av)4|²)/8}^((1/2))

Each of Lm and LM are determined by adding a product of the determinedvar2 and a suitable adjusting value adj2 in a similar manner to that ofthe second embodiment.

-   -   Lm=g_(av)4−var2*adj2    -   LM=g_(av)4+var2*adj2

Note that when the adjusting value adj2 is set to “2” as in the case ofthe second embodiment, it has been known that about 95% of the signalvalues of the video signals varied due to measurement errors takingvalues between the predetermined values Lm and LM. Accordingly, when thesignal value of the video signal of the target pixel G22 varies due to ameasurement error, a possibility that it is judged that there is adefect is reduced, and a more suitable noise reduction processing isenabled.

Alternatively, in the first and second embodiments, the noise reductionprocessing is performed by use of all of the external peripheral pixelsarranged at the positions which are one pixel away from the targetpixel. However, the noise reduction circuit 3 may perform the noisereduction processing by use of the external peripheral pixels which havebeen judged to be free from a defect. In this case, the signalreading-out unit 32 b shown in FIG. 3 and in FIG. 8 functions also as anexternal defect judgment unit for judging whether the defect hasoccurred in the external peripheral pixels. On this occasion, the signalreading-out unit 32 b judges that the defect has not occurred when thesignal value of the video signal of the external peripheral pixel whichit has read out is in range from the third predetermined value and thefourth predetermined value, as in the case of the defect judgment unit36. In addition, the signal reading-out unit 32 b outputs the signalvalues of the video signals of the external peripheral pixels which havebeen judged to be free from the defect. Accordingly, the signal valuesof the video signals of the external peripheral pixels are supplied tothe difference calculator 33 a, the average value calculators 37 athrough 37 c, the threshold value calculator 40 and the predeterminedvalue calculator 41, and are used for processing to reduce the noises ofthe target pixel. Note that the third predetermined value may be a valueequal to the first predetermined value, that is, Lm, and the fourthpredetermined value may be a value equal to the second predeterminedvalue, that is, LM. In addition, the noise reduction circuit 3 mayinclude an external defect judgment unit in addition to the signalreading-out unit 32 b.

According to the above described constitution, when a signal valueoutput as the signal value of the video signal of the target pixel iscalculated, the signal values of the video signals of any pixels inwhich a defect has occurred are not used. Accordingly, it is possible toperform the noise reduction processing more accurately.

Furthermore, in the first embodiment and the second embodiment, thoughthe internal peripheral pixels are used, it is acceptable that therelation between each of the external peripheral pixels and the targetpixel is used and the relations between the internal peripheral pixelsare not used. According to such a constitution, it is possible torealize the noise reduction device while suppressing cost more easily.

Furthermore, in the first embodiment and the second embodiment, thenoise reduction device and the noise reduction method are realized byuse of the noise reduction circuit. However, the noise reduction deviceand the noise reduction method can be realized by installing a program,which has the same function as the noise reduction circuit, onto acomputer having a CPU and memory.

1. A noise reduction device reducing a noise superimposed on a videosignal including plural kinds of color signals, each of which is outputby a pixel, comprising: a first signal reading-out unit for reading outa video signal of a target pixel for which a noise reduction isperformed; a second signal reading-out unit for reading out videosignals of external peripheral pixels arranged around internalperipheral pixels arranged at positions closest to the target pixel, theexternal peripheral pixels being arranged at positions closest to thetarget pixel in radial directions from the target pixel and outputtingthe same color signals as the video signal of the target pixel; aselection controller for judging presence of a correlation between thevideo signal of the target pixel and a video signal of each of theexternal peripheral pixels based on a relation between a signal value ofthe video signal of the target pixel and a signal value of the videosignal of each of the external peripheral pixels; a signal calculatorfor calculating the signal value of the video signal of the target pixelby use of the signal values of the video signals of the externalperipheral pixels; and a selector for selecting the signal value of thevideo signal of the target pixel determined by the signal calculatorwhen it is judged by the selection controller that the number of videosignals of the external peripheral pixels having correlations with thevideo signal of the target pixel is large.
 2. The noise reduction deviceaccording to claim 1, further comprising a third signal reading-out unitfor reading out video signals of internal peripheral pixels arrangedaround the target pixel and at positions closest to the target pixel inradial directions from the target pixel, wherein the selectioncontroller judges presence of a correlation between the video signal ofthe target pixel and the video signal of each of the external peripheralpixels based on a relation between the signal value of the video signalof the target pixel and the signal value of the video signal of each ofthe external peripheral pixels and based on relations between signalvalues of the video signals of the internal peripheral pixels.
 3. Thenoise reduction device according to claim 2, when the number of theexternal peripheral pixels are provided by n (n: a natural number), then external peripheral pixels are first through n-th external peripheralpixels, the n internal peripheral pixels are first through n-th internalperipheral pixels, and a direction of the k-th (k: a natural numbersatisfying 1≦k≦n) external peripheral pixel with respect to the targetpixel and a direction of the k-th internal peripheral pixel with respectto the target pixel are the same, the device further comprising: a firstcorrelation value calculator for determining first to n-th maincorrelation values respectively indicating presence of correlationsbetween the respective video signals of the first through n-th externalperipheral pixels and the video signal of the target pixel, based onresults obtained by subtracting the signal value of the video signal ofthe target pixel, which has been read out by the first signalreading-out unit, from the respective signal values of the video signalsof the first through n-th external peripheral pixels, which have beenread out by the second signal reading-out unit; and a second correlationvalue calculator for determining first through n-th sub-correlationvalues respectively indicating presence of correlations between therespective video signals of the first through n-th external peripheralpixels and the video signal of the target pixel, based on resultsobtained by subtracting the signal values of the video signals of theinternal peripheral pixels existing at a position symmetrical to therespective first through n-th internal peripheral pixels around thetarget pixel, from the signal values of the video signals of therespective first through n-th internal peripheral pixels, which havebeen read out by the second signal reading-out unit, wherein theselection controller judges presence of a correlation between the videosignal of each of the first through n-th external peripheral pixels andthe video signal of the target pixel based on the first to n-th maincorrelation values and the first through n-th sub-correlation values. 4.The noise reduction device according to claim 3, wherein the firstthrough n-th main correlation values and the first through n-thsub-correlation values are respectively binarized values; the firstcorrelation value calculator sets the value of the k-th main correlationvalue to be a value indicative of presence of a correlation with thevideo signal of the target pixel, when a value obtained by subtractingthe signal value of the video signal of the target pixel from the signalvalue of the video signal of the k-th external peripheral pixel is in arange from a first threshold value to a second threshold value; thesecond correlation value calculator sets the value of the k-thsub-correlation value to be a value indicative of presence of acorrelation with the video signal of the target pixel, when a valueobtained by subtracting the signal value of the video signal of theinternal peripheral pixel existing at a position symmetrical to the k-thinternal peripheral pixel around the target pixel from the signal valueof the video signal of the k-th internal peripheral pixel is in a rangefrom a third threshold value to a fourth threshold value.
 5. The noisereduction device according to claim 4, further comprising: a thresholdvalue calculator for determining the first and second threshold valuesby use of the signal values of the video signals of the externalperipheral pixels.
 6. The noise reduction device according to claim 5,wherein the threshold value calculator determines the first and secondthreshold values by use of an average value of the signal values of thevideo signals of the external peripheral pixels.
 7. The noise reductiondevice according to claim 3, wherein the main correlation valueindicating presence of a correlation with the video signal of the targetpixel is defined as a correlated main correlation value, and thesub-correlation value indicating presence of a correlation with thevideo signal of the target pixel is defined as a correlatedsub-correlation value, the selection controller compares the totalnumber of the correlated main correlation values with a firstpredetermined number, the selection controller judges the number of thevideo signals of the external peripheral pixels having correlations withthe video signal of the target pixel to be small when the total numberof the correlated main correlation values is smaller than the firstpredetermined number, and the selection controller judges the number ofthe video signals of the external peripheral pixels having correlationswith the video signal of the target pixel to be large when the totalnumber of the correlated main correlation value is the firstpredetermined value or more.
 8. The noise reduction device according toclaim 7, wherein the selection controller compares the total number ofthe correlated main correlation values with a second predeterminednumber larger than the first predetermined number when the total numberof the correlated main correlation values is the first predeterminednumber or more, when the selection controller judges that the totalnumber of the correlated main correlation values is in a range from thefirst predetermined number to the second predetermined number inclusive,the selector selects a weighted average value of the video signals ofthe external peripheral pixels corresponding to the correlated maincorrelation values calculated by the signal calculator; and when theselection controller judges that the total number of the correlated maincorrelated values is larger than the second predetermined number, theselector selects a weighted average value of the video signals of theexternal peripheral pixels corresponding to both of the correlated maincorrelation values and the correlated sub-correlated values calculatedby the signal calculator.
 9. The noise reduction device according toclaim 1, wherein, when the selection controller judges that the numberof the video signals of the external peripheral pixels havingcorrelations with the video signal of the target pixel is small, theselector selects the signal value of the video signal of the targetpixel, which has been read out by the first signal reading-out unit. 10.The noise reduction device according to claim 7, wherein when theselection controller judges that the number of the external peripheralpixels having correlations with the video signal of the target pixel issmall, the selection controller compares the signal value of the videosignal of the target pixel, which has been read out by the first signalreading-out unit, with a first predetermined value and a secondpredetermined value, when the selection controller judges that thesignal value of the video signal of the target pixel is in either of arange smaller than the first predetermined value and a range larger thanthe second predetermined value, the selector selects a weighted averagevalue of the video signals of the external peripheral pixelscorresponding to the correlated sub-correlation value calculated by thesignal calculator, and when the selection controller judges that thesignal value of the video signal of the target pixel is in a range froma first predetermined value to a second predetermined value inclusive,the selector selects the signal value of the video signal of the targetpixel which has been read out by the first signal reading-out unit. 11.The noise reduction device according to claim 10, further comprising apredetermined value calculator for determining the first and secondpredetermined values by use of the signal values of the video signals ofthe external peripheral pixels.
 12. The noise reduction device accordingto claim 11, wherein the predetermined value calculator determines thefirst and second predetermined values by use of a value indicating amagnitude of variations of the signal values of the video signals of theexternal peripheral pixels.
 13. A noise reduction device reducing anoise superimposed on a video signal including plural kinds of colorsignals, each of which is output by a pixel, comprising: a selectioncontroller for judging presence of a correlation between a video signalof the target pixel and a video signal of each of the externalperipheral pixels based on a relation between a signal value of thevideo signal of the target pixel and a signal value of the video signalof each of external peripheral pixels arranged at positions close to thetarget pixel among pixels outputting the same color signals as thetarget pixel for which a noise reduction is performed,; and a selectorfor selecting a signal value output as the signal value of the videosignal of the target pixel from a signal value calculated by use of thesignal values of the video signals of the external peripheral pixels andthe signal value of the video signal of the target pixel, in accordancewith results judged by the selection controller.
 14. The noise reductiondevice according to claim 13, wherein the selection controller judgespresence of correlations between the video signal of the target pixeland the video signal of each of the external peripheral pixels based ona relation between the signal value of the video signal of the targetpixel and the signal value of the video signal of each of the externalperipheral pixels and based on relations between respective signalvalues of video signals of internal peripheral pixels arranged atpositions adjacent to the target pixel.
 15. The noise reduction deviceaccording to any one of claims 1 and 13, further comprising an externaldefect judgment unit for judging whether a defect has occurred in eachof the external peripheral pixels, and wherein the external peripheralpixels used for reducing the noises of the target pixel are externalperipheral pixels judged by the external defect judgment unit to bethose where the defect has not occurred.
 16. The noise reduction deviceaccording to claim 15, wherein the external defect judgment unit judgesthat the defect has not occurred when the signal value of the videosignal of each of the external peripheral pixels is in a range from athird predetermined value to a fourth predetermined value inclusive. 17.A noise reduction method which reduces a noise superimposed on a videosignal including plural kinds of color signals, each of which is outputby a pixel, comprising: a step of reading out a video signal of a targetpixel for which a noise reduction is performed; a step of reading outvideo signals of external peripheral pixels arranged around internalperipheral pixels arranged at positions closest to the target pixel, theexternal peripheral pixels being arranged at positions closest to thetarget pixel in radial directions from the target pixel and outputtingthe same color signals as the video signal of the target pixel; a stepof judging presence of a correlation between the video signal of thetarget pixel and a video signal of each of the external peripheralpixels based on a relation between a signal value of the video signal ofthe target pixel and a signal value of the video signal of each of theexternal peripheral pixels; a step of calculating the signal value ofthe video signal of the target pixel by use of the signal values of thevideo signals of the external peripheral pixels; and a step of selectingthe signal value of the video signal of the target pixel calculated byuse of the signal values of the video signals of the external peripheralpixels when it is judged that the number of the video signals of theexternal peripheral pixels having correlations with the video signal ofthe target pixel is large.
 18. A noise reduction method which reduces anoise superimposed on a video signal including plural kinds of colorsignals, each of which is output by a pixel, comprising: a step ofjudging presence of a correlation between a video signal of a targetpixel and a video signal of each of the external peripheral pixels basedon a relation between a signal value of the video signal of the targetpixel and a signal value of the video signal of each of the externalperipheral pixels arranged at positions close to the target pixel amongpixels outputting the same color signals as the target pixel for which anoise reduction is performed; and a step of selecting a signal valueoutput as the signal value of the video signal of the target pixel froma signal value calculated by use of the signal values of the videosignals of the external peripheral pixels and the signal value of thevideo signal of the target pixel, in accordance with results judged bythe selection controller.
 19. An image pickup apparatus, comprising: asolid state image pickup device having plural kinds of color filtersprovided on surfaces of pixels and outputting video signals which becomeplural kinds of color signals; and the noise reduction device defined inany one of claims 1 and 13, to which the video signals from the solidstate image pickup device are input.